Can't see any fleet numbers but I think the vehicle is an NS type, working out of Camberwell on route 40. The driver, the chap in the white dustcoat, was my great uncle Ted. The photograph came to light when my grandfather, Ted's brother in law, passed away 13 years ago. As you can see I managed to get a copy made as the photograph is now in the USA, as his son quite rightly wanted it. 

See Richard's site of Cambridge buses at:- http://www.the-busman.co.uk

Geoff Bannister says:-
The vehicle in question is actually an S class vehicle. A number of clues lead to that decision. To my knowledge, no NS class vehicles carried XL registrations. Secondly, the NS had a very narrow window immediately behind the driver's cab and the driver's dash was different. Finally, the NS was the first class desgined to have a top cover and had a lower floor line. On an NS, the bottom line of the body panelling is roughly level with the wheel centres ahile on an S, the same panel line is level with the top of the wheel rim.